The present invention relates to positive displacement hydraulic pumps of the type which may be utilized in vehicle power steering systems, and more particularly, to such pumps which include a flow control valve assembly to bypass excess output fluid.
In vehicle power steering systems, positive displacement, rotary pumps are ordinarily employed for supplying pressurized fluid to a power steering control device, with the pump being driven by the vehicle engine, and operating over a widely varying range of speeds.
Hydraulic pumps of the type to which the present invention relates include a housing defining a pumping chamber and a pumping element disposed therein. The housing defines an inlet port communicating with expanding pumping chambers, and an outlet port communicating with contracting pumping chambers. The housing further includes a discharge port which may be connected to a fluid operated device, such as the power steering control unit.
In pumps of the type described above, the volume of fluid flowing from the outlet port increases proportionally as the speed of the engine increases. Therefore, in conventional power steering systems, a flow control valve assembly is disposed in the flow path between the outlet port and the discharge port. The flow control valve is typically responsive to a fluid pressure differential generated across a metering orifice through which discharge fluid flows. The flow control valve assembly establishes a predetermined maximum flow rate from the discharge port, with all excess fluid being bypassed back to the inlet port.
Typically, the flow control valve assembly includes a spool valve movably disposed in a valve bore, and including a spool land disposed between the discharge fluid path and a bypass port, through which excess fluid is recirculated. Below the flow control setting (e.g., 2.5 gpm), the flow control spool land prevents communication from the discharge fluid path to the bypass flow path. When the fluid pressure in the discharge fluid path is relatively high (e.g., 2000 psi), there is a tendency for substantial fluid leakage from the discharge fluid path, through the clearance between the spool land and valve bore, to the bypass flow path. This leakage results in a loss of flow from the discharge port, thus reducing the volumetric efficiency of the pump, and the hydraulic horsepower delivered to the fluid operated device.